1. Field of the Invention
The present invention relates to a method for producing an organic thin-film device by forming a conductive or insulating thin-film on a functional organic film, and more particularly to a method for producing an organic thin-film device by use of a facing-targets-type sputtering apparatus configured such that a pair of facing targets are disposed a predetermined distance away from each other and such that a magnetic field extending between the targets from one target to the other is generated in such a manner as to laterally surround a space provided between the facing targets (the space is hereinafter called a confinement space), to thereby confine plasma within the confinement space and form a film on a substrate disposed at a position beside the confinement space under vacuum.
2. Description of the Related Art
In recent years, organic electroluminescent devices (hereinafter simply referred to as xe2x80x9corganic EL device(s)xe2x80x9d), which are a type of organic thin-film device, have become of interest as light-emitting thin-film displays, and extensive studies have been conducted in order to put these thin-film displays into practical use. A typical organic electroluminescent device includes a positive electrode of a transparent electrode constituted by indium-tin oxide (ITO) or a similar substance formed on, for example, a glass or plastic transparent substrate; an organic layer formed from a hole-transporting organic compound such as triphenyldiamine (TPD), the layer being formed on the transparent electrode through a conventional thermal evaporation method; a light-emitting layer formed from a fluorescent substance such as 8-hydroxyquinoline aluminum (Alq3), the layer being formed on the organic layer; and a negative electrode of a metal of small work function such as Mg, the electrode being formed on the light-emitting layer. Such organic EL devices have become of interest, since they provide a considerably high luminance of 100 to 10,000 cd/m2 when operated at a low driving voltage of about 10 V.
Many studies have been performed with an aim to improve properties of such organic EL devices. Particularly, various methods for forming a negative electrode on an organic layer such as a light-emitting layer have been proposed, the method being considered one of key technologies for improving device properties and productivity and for attaining reliable production.
For example, Japanese Laid-Open Patent Publication (kokai) No. H08-250284 discloses a method for forming a negative electrode, through sputtering, on a substrate disposed so as to face a target. The publication describes the following. A negative electrode formed through a typical method of thermal evaporation raises a problem in that an oxide of a metal serving as the raw material of the electrode is generated in the interface between the electrode and an organic layer or in the electrode, to thereby vary electron-injecting properties, resulting in failure to obtain desired device properties. In addition, because of poor adhesion between the electrode and the organic layer, emission intensity is lowered with passage of voltage application time, and the electrode is exfoliated and dissipated, resulting in difficulty in forming a reliable device. In contrast, in the case where a negative electrode thin film is formed through sputtering, adhesion between the electrode and an organic layer is improved as compared with the case where a negative electrode is formed through a thermal evaporation method. Furthermore, an oxide layer formed on the target can be removed under vacuum through pre-sputtering, and water or oxygen adsorbed on the surface of the organic layer can be removed through reverse-sputtering, attaining satisfactory formation of a clean electrode on an organic layer, leading to the production of a reliable organic EL device.
Japanese Laid-Open Patent Publication (kokai) No. H10-255987 discloses a method for producing an organic EL device including an electrode formed by means of a facing-targets-type sputtering method in which a film is formed on a substrate disposed at a position beside a space provided between a pair of facing targets. In the course of production of the organic EL device, a negative electrode is formed on an organic film of the organic EL device by use of a facing-targets-type sputtering apparatus. The apparatus includes a pair of parallel facing targets disposed a predetermined distance away from each other; magnetic-field generation means which generates a magnetic field in a direction substantially perpendicular to the targets; and a shield disposed so as to cover a portion of each target other than the facing surface. In the apparatus, a substrate is disposed at a position beside a space provided between the facing targets, power is applied between the target and the shield, and the thus-generated plasma is confined within the space, to thereby form a film on the substrate.
The publication describes the following. In the case of conventional sputtering method such as the aforementioned magnetron-type sputtering method, in which a substrate and a target are disposed so as to face each other, secondary electrons generated from the surface of the target and sputtered particles of high kinetic energy; specifically, large amounts of secondary electrons and ionized sputtered particles, impinge on an organic layer, to thereby physically break the organic layer. As a result, the electrostatic breakdown voltage of an organic EL device is lowered, and application of voltage between negative and positive electrodes may cause leakage of current. Alternatively, the device may fail to function, as a result of breakage associated with an increase in temperature. Furthermore, driving voltage increases and luminance decreases, to thereby deteriorate EL properties.
In contrast, in the aforementioned facing-targets-type sputtering apparatus, a plasma generation region and a film formation region are completely separated from each other, and a film is formed in a plasma-free-like state. Therefore, a substrate is substantially not exposed to plasma, and film formation is carried out such that an organic layer and a film deposited on the layer are not damaged by high-energy particles. As a result, problems arising from the aforementioned conventional sputtering method are solved, and the resultant organic EL device has a high luminance at an initial stage, and the time until the luminance becomes half increases. Furthermore, the device has very small amounts of dark spots at an initial stage, and generation and growth of dark spots after operation are reduced.
A conventional facing-targets-type sputtering apparatus will next be described. The sputtering apparatus is disclosed in Japanese Publication of Examined Patent Application (kokoku) Nos. S63-20303, S63-20304, and S62-14633, and has the following basic configuration as shown in FIG. 1. A pair of targets 110a and 110b are disposed a predetermined distance away from each other within a vacuum chamber 10 having a chamber wall 11, thereby defining a confinement space 120 therebetween. Permanent magnets 130a and 130b serving as magnetic-field generation means are disposed behind the corresponding targets 110a and 110b in order to generate a magnetic field which extends in a direction perpendicular to the targets 110a and 110b and whose flux uniformly surrounds the confinement space 120. A substrate holder 21 disposed at a position beside the confinement space 120 holds a substrate 20 such that the substrate 20 faces the confinement space 120. Reference numerals 140a and 140b represent shields for protecting from sputtering portions of target units 100a and 100b other than the front surfaces of the targets 110a and 110b. 
After the vacuum chamber 10 is evacuated through an evacuation port 30 by means of an unillustrated evacuation system, a sputtering gas, such as argon, is introduced into the vacuum chamber 10 through a gas inlet 40 by means of an unillustrated gas introduction means. As shown in FIG. 1, a DC sputtering power supply 50 supplies sputtering power to the apparatus while the shields 140a and 140b; i.e., the vacuum chamber 10, serve as an anode (ground) and the targets 110a and 110b serve as a cathode. Thus, sputtering plasma is generated while being confined within the confinement space 120 by means of the perpendicular magnetic field. The sputtering plasma effects sputtering of the targets 110a and 110b, thereby forming on the substrate 20 a thin film whose composition corresponds to that of the targets 110a and 110b. 
In use of the apparatus, since the magnetic field extends in the direction extending between the targets 110a and 110b; i.e., perpendicular to the targets 110a and 110b, high-energy electrons are confined within the confinement space 120 so as to generate sputtering plasma. The sputtering plasma accelerates ionization of the sputtering gas, thereby increasing a sputtering rate and thus forming a film at high rate. In contrast to a typical conventional magnetron-type sputtering method, in which a substrate is disposed in opposition to a target, the substrate 20 is disposed at a position beside the targets 110a and 110b. Accordingly, ions and electrons impinging on the substrate 20 are greatly reduced. Also, thermal radiation from the targets 110a and 110b to the substrate 20 is low, so that an increase in substrate temperature becomes small. Thus, a film can be formed at low temperature.
However, the present inventors have found that, in the case of an organic EL device including a negative electrode formed by means of the method disclosed in Japanese Laid-Open Patent Publication (kokai) No. H10-255987 and by use of the conventional facing-targets-type sputtering apparatus shown in FIG. 1, in order to obtain emission having a certain luminance at an initial stage, a higher voltage is required as compared with the case of an organic EL device including a negative electrode formed through a conventional thermal evaporation method.
Meanwhile, in order to put organic EL devices into practical use, there has been demand for a method for further improving emission properties of organic EL devices and for mass-producing organic EL devices reliably.
The aforementioned driving voltage increase is attributed to the below-described deterioration of properties of a functional organic layer, such as electron-transporting property and light-emitting property, during formation of an electrode. The driving voltage increase is a common problem among organic thin-film devices having a structure including a functional organic layer and a thin-film layer such as an electrode formed on the organic layer. Also, the driving voltage increase becomes a problem when organic thin-film devices such as organic EL devices, organic FET devices, and organic solar batteries are put into practical use.
An object of the present invention is to provide a method for producing an organic thin-film device, wherein a thin-film layer is formed on a functional organic layer without imparting substantially damage to the functional organic layer.
Another object of the present invention is to provide a method for forming a high-quality thin-film layer such as a metallic film on a functional organic layer.
Still another object of the present invention is to provide an organic EL device which emits light of a certain luminance at a lower driving voltage.
Still another object of the present invention is to provide an organic EL device which emits light of higher luminance.
Still another object of the present invention is to provide a method for producing an organic EL device reliably exhibiting excellent emission properties at high productivity.
In order to achieve the above objects, the present invention provides a method for producing an organic thin-film device comprising a functional organic layer formed from an organic compound and a thin-film layer laminated on the organic layer, which method comprises forming the thin-film layer, by use of a facing-targets-type sputtering apparatus including a pair of facing targets disposed a predetermined distance away from each other; an electron reflection electrode disposed at the periphery of each target so as to face a confinement space provided between the paired targets; and magnetic field generation means disposed at the lateral sides of each target, wherein the magnetic field generation means generates a magnetic field extending from one target to the other so as to surround a confinement space provided between the paired targets, as well as a magnetic field having a portion parallel to the surface of each target in the vicinity of a peripheral edge portion of the target.
The present invention has been accomplished as described below. The aforementioned driving voltage increase in the organic EL devices produced through the method described in Japanese Laid-Open Patent Publication (kokai) No. H10-255987 by use of the conventional facing-targets-type sputtering apparatus is considered to be attributed to an insulating layer; i.e., a property-deteriorating layer which is generated in the interface between the organic layer and the negative electrode. The insulating layer is generated for the reasons described below. The atomic bonding energy between atoms constituting organic substances contained in the organic layer provided below the negative electrode is as follows: Cxe2x80x94C bonding energy (348 kJ/mol) and Cxe2x80x94H bonding energy (412 kJ/mol). Since 96.5 kJ/mol corresponds to 1 eV, such a bonding energy becomes several eV.
Meanwhile, in the conventional facing-targets-type sputtering method, since discharge voltage is high during film formation, recoiled argon and electrons having an energy several times the aforementioned bonding energy impinge on the organic layer, and the recoiled argon and electrons break organic compounds in the surface of the organic layer, resulting in formation of an insulating film in the interface between the organic layer and the negative electrode.
The present inventors have performed extensive studies, and have found that, as described below, the objects of the present invention can be achieved by use of the facing-targets-type sputtering apparatus having the aforementioned structure. The present invention has been accomplished on the basis of this finding.
Specifically, the present inventors have found that, in the facing-targets-type sputtering apparatusxe2x80x94in which a magnetic field parallel to facing targets is generated around the targets, while a magnetic field perpendicular to the facing targets is generated as in the case of the aforementioned conventional facing-targets-type sputtering apparatus, and electron reflection electrodes are provided on the periphery of each target, to thereby confine plasma in a confinement space provided between the targetsxe2x80x94plasma is more effectively confined in the confinement space and a film is formed at lower gas pressure and discharge voltage as compared with the case of the conventional sputtering apparatus in which merely a perpendicular magnetic field is generated. Also, the present inventors have found that, when a negative electrode is formed at low gas pressure and discharge voltage, as described below in Experiments, the resultant organic EL device exhibits initial emission properties such as driving voltage and luminance similar to those with a negative electrode formed through a conventional thermal evaporation method, and that the maximum luminance of the EL device is improved considerably; i.e., the maximum luminance becomes three to five times that of an organic EL device produced through the thermal evaporation method.
Therefore, a large current can be supplied, in a film thickness direction, to a negative electrode formed by use of the sputtering apparatus of the present invention, whereas such a large current cannot be supplied to a negative electrode formed through the conventional thermal evaporation method, due to damage imparted to an organic layer lying under the electrode. According to the present invention, a thin film having no defects can be formed on an organic thin film without imparting any damage to the surface and inside of the organic thin film.
The reasons why such excellent emission properties and thin film are obtained by the present invention have not been elucidated, but are considered to be as follows. Since film formation can be performed at low gas pressure and discharge voltage in the facing-targets-type sputtering apparatus of the present invention unlike the case of the conventional facing-targets-type sputtering apparatus, the kinetic energy of sputtering particles used for film formation can be regulated at such a level that the particles can be diffused on the surface of a deposition film so as not to cause any lattice defect, and the number of sputtering gas particles and electrons which impinge on the organic layer serving as a substrate, the particles and electrons having a high energy so as to break bonding of atoms constituting an organic compound comprising the organic layer, can be regulated to such a level that substantially no problems arise. Therefore, imparting of damage to the surface of the organic layer can be prevented, and films exhibiting excellent durability and having no defects can be continuously formed on the organic layer. In other words, according to the present invention, a thin film having high quality can be formed on the organic layer without imparting any damage to the organic layer, since sputtered particles used for film formation have the kinetic energy of a level that the particles can be diffused on the surface of a deposition film so as not to cause any lattice defect.
In the present invention, the gas pressure and discharge voltage are preferably determined such that an excellent electrode is formed on an organic layer so as not to impart any damage to an organic compound comprising the organic layer. Preferably, the gas pressure and discharge voltage are determined on the basis of experiments, since they are varied with the type of the organic compound and the material of the electrode. Specifically, as described below in Experiments in which organic EL devices are produced, the gas pressure is preferably 0.3 Pa or less, and the discharge voltage is preferably 300 V (absolute value) or less. The reason for this is because, when the gas pressure is higher than 0.3 Pa, the amount of a sputtering gas component such as argon contained in a thin film increases, resulting in deterioration of the quality of the thin film, and when the discharge voltage is higher than 300 V, the energy of recoiled argon and xcex3-electrons, which are generated during film formation, increases, and thus an organic film tends to be damaged. However, a very low discharge voltage is not preferable, from the viewpoint of ensuring sputtered particles used for film formation have the kinetic energy of a level that any lattice defect is not caused in the film. Therefore, the discharge voltage is preferably about 100 V (absolute value) or more.
In the facing-targets-type sputtering apparatus employed in the present invention, preferably, as has been proposed by one of the present inventors in Japanese Patent Application No. 2000-36965 (U.S. patent application Ser. No. 09/998,235), sputtering is carried out by use of AC-DC power containing a DC component and a high-frequency component, since a discharge voltage can be regulated over the wide range, particularly in the range of low voltage, as compared with the case of a conventional sputtering apparatus, and a discharge voltage suitable for various materials can be determined within a range such that a functional organic layer sustains substantially no damage.
It has been confirmed that, in the conventional facing-targets-type sputtering apparatus described in Japanese Laid-Open Patent Publication (kokai) No. H10-255987, even when AC-DC power containing a DC component and a high-frequency component is supplied, a discharge voltage cannot be reduced, and plasma is not confined between facing targets and is spread over a substrate, resulting in damage to an organic layer.
Although the present invention will next be described in detail with reference to embodiments in which the invention is applied to organic EL devices, it is apparent that the invention is not limited to the embodiments and may be applied to various organic thin-film devices having a structure including a functional organic layer and a thin-film layer formed on the organic layer.
As used herein, the term xe2x80x9cfunctional organic layerxe2x80x9d refers to a functional layer which contains an organic compound exhibiting predetermined properties, specifically electromagnetic properties such as electron-transporting property, hole-transporting property, light-emitting property, conductivity, magnetic property, and insulating property, and which itself exhibits such predetermined properties. No particular limitation is imposed on the thin-film layer to which the present invention can be applied, the layer being formed on the functional organic layer. The present invention may be applied to an electrode layer of a metal or a conductive oxide which must be formed through sputtering in consideration of the quality of the layer; or may be applied to an inorganic layer such as an insulating protective layer of a metal oxide or a metal nitride. Of these, the present invention is preferably applied to an electrode layer necessary for a good interface which is bonded to the functional organic layer.
Specific examples of the aforementioned organic thin-film devices include organic EL devices, organic solar battery devices, organic rectifier devices, and organic field effect transistors (FETs). The structures of an organic solar battery device, an organic rectifier device, and organic FET are briefly described below.
An organic solar battery device has a structure such that two electrodes (at least one of the electrodes is a transparent electrode) sandwich an organic thin film having a two-layer structure including an electron-transporting layer and a charge-generating layer formed from dye molecules which absorb visible rays to thereby generate electrons and holes, or an organic thin film having a structure of three or more layers including a hole-transporting layer, an electron-transporting layer, and the aforementioned charge-generating layer provided between the hole-transporting layer and electron-transporting layer. In the device having the two-layer structure or the structure of three or more layers, an important feature is to prevent recombination of generated electrons and holes to thereby allow electron-hole separation to proceed effectively, in order to increase photoelectric conversion efficiency.
An organic rectifier device has a structure in which two electrodes sandwich an organic thin film including a hole-transporting layer (p-type semiconductor) and an electron-transporting layer (n-type semiconductor). The device exerts rectification effect, since a barrier is formed in the junction portion between the hole-transporting layer and electron-transporting layer, similar to the case of the pn junction of an inorganic semiconductor. When the hole-transporting layer and electron-transporting layer are doped with small amounts of acceptor molecules and donor molecules, respectively, current density can be increased.
An organic FET is a gate-insulated transistor including an organic semiconductor such as a polythiophene derivative. In the FET, source and drain electrodes must be formed on the organic semiconductor. In the case where the FET includes a p-type organic semiconductor, when a negative voltage is applied to a gate electrode, holes are accumulated in the interface between the insulator and the semiconductor, and electrical conductivity increases in the vicinity of the interface. When a voltage lower than that applied to the source electrode is applied to the drain electrode, the thus-accumulated holes are injected from the source electrode into the semiconductor, to thereby cause current to flow.
The foregoing and other objects of the present invention, together with its novel features, will become more apparent from the following detailed description and with reference to the accompanying drawings.